Interdigitated strip line coupler



June 2, 1970 ,J. LANGE v 3,

' IINTERDIGITAL'IED' STRIP LINE cbuPLER Filed Dec. 30. 1968zlsh'eets-sheet- 1 3 dB QUADRATURE 3d8 QUADRATURE COUPLER COUPLER INPUT?L I I v GAIN moouuzs 3 or 4 STAGES Q I i OOK JT PUT I4 I I 1o I I 2oFIG.

OUTPUT Fl 3 PRIOR ART INVENTOR:

JULIUS LANGE FIG. "4

ATTORNEY June 2; 1970 a q J.LANGE v "3,516,024

INTERDIGI'I'ATED STRIP LINE COUPLER Filed Dec. 30, 1968' 2 Sheeji's-Sheet 2 2 I I i Z I I 4 an o" J v" 5g 0" "r *r "r 9 FIG-'5 30 I :0 I

INVENTOR:

JULIUS 'LANGE ATTORNEY United States Patent Office 3,516,024 PatentedJune 2, 1970 3,516,024 INTERDIGITATED STRIP LINE COUPLER Julius Lange,Dallas, Tex., assignor to Texas Instruments Incorporated, Dallas, Tex.,a corporation of Delaware Filed Dec. 30, 1968, Ser. No. 787,784

' Int. Cl. H01p /14 US. Cl. 333 12 Claims ABSTRACT OF THE DISCLOSURE Aninterdigitated coupler is fabricated from several strip line sectionswith alternate sections interconnected by crossover wires. Theinterdigitated strip line sections are arranged such that each sectionis on the order of a quarter wavelength long.

This invention relates to directional couplers and more particularly to'an interdigitated quadrature strip line coupler.

A directional quadrature coupler is a four port microwave junction withproperties such that a wave incident in port one couples power intoports two and three, but not into port four. Similarly, power incidentin port four couples into ports two and three but not into port one.Thus, ports one and four are uncoupled. A wave incident in port two orthree couples power into ports one and four only, thus ports two andthree are also uncoupled. If three of the four ports are terminated inmatched loads, the fourth port appears terminated in a matched load, andan incident wave in this port will not be reflected.

In one application, directional couplers are used to divide the powerbetween two amplifiers to reduce the terminal voltage standing waveratio (VSWR) and increase circuit design flexibility. Ideally, a couplershould be lossless with exactly 3-db power division across the band.Heretofore, a low loss, low VSWR 3-db strip line coupler has beendifficult to achieve due to line spacing and fabrication tolerances.Both the branch line coupler and the non-interdigitated edge-coupledcoupler are among those which are diflicult to fabricate due to thetolerance requirements. Further, branch line couplers are not capable ofsimultaneously meeting the VSWR and bandwidth objectives of mostcoupling requirements within a reasonable number of sections. The tandemcoupler also has severe limitations and, in addition, is rather largedimensionally. In addition, both these configurations have a narrowbandwidth and require a much larger substrate than singlesectionedcoupled line couplers.

Tight coupling in directional couplers for microwave integrated circuitshas also been achieved by means of broadside coupling and re-entrantsections. Both these configurations, however, require multilayercircuitry,

which isjyery difficult to build on ceramic, and virtually impossible inthe case of monolithic integrated circuits.

An object of the present invention is to provide a low loss quadraturecoupler. Another object of this invention is to provide a wide bandwidthquadrature coupler. Still another object of this invention is to providea quadrature coupler having reasonable fabrication tolerances. A furtherobject of this invention is to provide a quadrature coupler havinginterdigitated strip line sections.

In accordance with the present invention, several strip line sectionsare formed on a substrate in an interdigitated pattern. Alternate linesections are tied together by jumper wires bonded thereto. The effectivelength of each line section is on the order of a quarter wavelength atthe bandwidth center frequency. The design of the input ports issymmetrical with the interdigitated line sections, thus improving thecoupling characterstics.

In accordance with a specific embodiment of the invention, ports one andthree of an interdigitated 3-db coupler are formed integral withindividual line sections. These sections are interconnected by multiplebonding wires to reduce parasitic inductance. Ports two and four areinterconnected by a single quarter wavelength section and formedintegral with a line section one-half a quarter wavelength long.Multiple bonding wires interconnect the short line sections to thequarter wavelength section between ports two and four. The line sectionsare fabricated in an interdigitated pattern by a vapor depositionprocess.

A more complete understanding of the invention and its advantages willbe apparent from the specification and claims and from the accompanyingdrawings illustrative of the invention.

Referring to the drawings:

FIG. 1 is a block diagram of an amplifier employing quadrature couplersat the input and output;

FIG. 2 is a schematic diagram of a typical single amplification stagefor the amplifier of FIG. 1;

FIG. 3 is an illustration of a three section cascade directional couplerof the type found in the prior art;

FIG. 4 is an illustration of an interdigitated quadrature coupler inaccordance with the present invention;

FIG. 5 is a plot of transmission magnitude in db versus frequency ingigahertz for the coupler of FIG. 4; and

FIG. 6 illustrates an alternate embodiment of an interdigitated couplerin accordance with the present invention.

Referring to the drawings, in FIG. 1 there is illustrated a completeamplifier consisting of two 3-db quadrature couplers and two gainmodules. An input coupler having four ports has port one connected to aninput terminal 12, port four connected to ground through a resistor 1-4,and ports two and three coupled to gain modules 16 and 18, respectively.An output coupler 20 also having four ports has ports two and threeconnected to the output of the gain modules 16 and 18, respectively,port one connected to ground through a resistor 22, and port fourconnected to an output terminal 24.

The gain modules may employ three or four stages of amplification tomeet the desired amplifier objectives.

Referring to FIG. 2, there is shown schematically an amplifier stage ofthe type employed in the gain modules 16 and 18. An input terminal 26connects to a port of the input coupler 10 or to a precedingamplification stage. A coupling capacitor 28 connects the input terminalto the base electrode of a transistor 30 which is also connected to abias choke 32. The choke 32 is connected to ground through a capacitor34 and to a source of direct current (not shown) connected to theterminal 36. Transistor 30 also includes an emitter electrode connectedto a source of direct current (not shown) at terminal 38 through aresistor '40. The emitter electrode of the transistor 30 also connectsto ground through a capacitor 42. The output circuit of the amplifierincludes an L-section trans former 44 connected to the collectorelectrode of the transistor 30 and to a capacitor 46. Transformer 44, inaddition to connecting to the output terminal 48, also connects to abias choke 50.

Preferably, each amplifier stage of the gain modules .16 and '18 will befabricated by means of strip line techniques. The chokes 32 and 50 aremeandered lines in strip line configuration, each a quarter wavelengthlong at the bandwidth center frequency. The transistor 30 may befabricated on a chip and inserted into the strip line circuitry.

Techniques for fabricating amplifier stages of the type illustrated inFIG. 2 have been developed such that each unit requires only a minimumof space. However, prior art couplers of the type illustrated in FIG. 3require considerably more space. In addition, prior art couplers of thetype illustrated are difficult to fabricate due to line spacing andtolerances. The coupler of FIG. 3 includes two outer sections 52 and 54forming a region of loose coupling each one quarter wavelength long.Typically, the two outer sections 52 and 54 have 17.2-db coupling, whichcan be realized with edge coupled lines 8 mils wide spaced 16 mils aparton a 20 mil ceramic substrate. If the two outer sections have a 17.2-dbcoupling, then a center section 56 requires 1.76 db coupling to producean overall 3-db coupler. The tight coupled center section 56 is againone quarter wavelength long. As illustrated, the section '56 has twospaced lines for edge coupling. The width and spacing of such lines isdifiicult to calculate and almost impossible to build. In an attempt touse couplers of the type illustrated in FIG. 3, broadside coupling hasbeen considered for the section 56. However, severe problems areencountered in trying to keep the lines overlapped for the completequarter wavelength, and depositing the very thin ceramic for the linespacing.

With an interdigitated coupler of the type illustrated in FIG. 4, theoverall area of the coupler can be reduced to one third that illustratedin FIG. 3, and fabrication procedures are simplified considerably.Referring to FIG. 4, a first line section 58 terminates at a port 60(port one) and a second line section 62 terminates at a port 64 (portthree). Ports one and three are thus similar and symmetrical. A thirdline section 66 terminates at one end at a port 68 (port two) and at theother end at port 70 (port four). Ports 68 and 70 are also formedintegral with line sections 72 and 74, respectively, which are half aslong as the section 66. Thus, the ports 68 and 70 are symmetrical, andthe entire coupler is symmetrical. Symmetry in a coupler of the typeillustrated improves the coupling efficiency by lowering losses.

Line sections 58, 62, 66, 72, and 74 form an interdigitated pattern.Alternate line sections are interconnected by crossover wires. Thus,section 58 is connected to section 62 by means of a group of threecrossover wires 76 at port 60 and a group of three crossover wires 78 atport 64. Line section 66 connects to the line section 72 by a group ofthree crossover wires 80 and to the line section 74 by means of a groupof three crossover wires 82. Multiple bonding wires for the crossoversare more easily implemented than single crossover wires and reduce theparasitic inductance associated with the bonding wires.

One model of the coupler shown in FIG. 4 was fabricated on a 42-milthick substrate of alumina (A1 with a ground plane covering one surface.The unglazed alumina was placed in a vacuum chamber and a thin gold filmdeposited on the side opposite the ground plane. Using photomasking andetch techniques, a first mask was formed over the thin gold film tooutline the strip line sections. The unwanted film was then removed byetching and a second mask formed on the substrate defining a strip linesection and the port areas. Gold was then plated over the exposed goldfilm areas through the second mask to a desired thickness. In onelaboratory model of an interdigitated coupler, the strip line sectionsare 4.5 mils wide and spaced 3 mils apart. It should be understood thatthe interdigitated coupler of the present invention is applicable tocircuits with two ground planes with either one or two layers ofdielectric.

Referring to FIG. 5, there is shown a plot of transmission magnitude indb versus frequency in gHz. for the model described above. For an inputcoupled to port one, power was transmitted to port three with anattenuation of about 3-db as shown by the curve 86. With the same inputconnected to port one, power was transmitted to port two with anattenuation of about 3.5-db, as indicated by curve 88. Both these curvesare for a frequency range of from 2 gHz. to 4 gHz. Between port one andport four, very little power was transmitted as indicated by the curve90 which is approximately 30 db down. It should be noted that the curvesof FIG. are not intended to indicate the peak performance of the couplerof FIG. 4. These curves were plotted from data taken from laboratoryexperiments.

Referring to Table 1, there is shown the complete response of aninterdigitated 3-db coupler of the present invention between 2.0 gHz.and 4.0 gHz. In addition to listing the data from which the curves ofFIG. 5 were plotted, Table 1 also lists insertion loss and outputimbalance. Again, this data is not intended to imply peak performance,but rather represents data obtained from laboratory experiments.

TABLE lr-RESPONSE OF INTERDIGI'IATED 3-db. COUPLER Insertion OutputDirect Coupled Direcloss Imbalance output Output tivity 18121 1Siz1Frequency 1813.1 18121 lSiil 181 1 18131 (gHz.) (db (db.) (db.) (db(db.)

Referring to FIG. 6, there is shown another embodiment of aninterdigitated coupler in accordance with the present invention. Striplines 92 and 94 are formed integral with port 96 (port three). Thesestrip lines are connected to port 98 (port two) by means of a crossoverwire 100 bonded to the strip lines and the port. A strip line 102 isformed integral with the port 104 (port four) and port 106 (port one).The port 104 is also formed integral with a strip line 108interdigitated with the strip lines 92 and 94 and connected to the port106 by means of a crossover wire 110. The strip lines 92, 94, 102, and108 are a quarter wavelength long at the bandwith center frequency.

A 3-db interdigitated quadrature coupler in the pattern of FIG. 6 wasfabricated on a 40 mil thick substrate of alumina. The design consistsof four interdigitated strip lines, each 4.5 mils wide, spaced 3.0 milsapart. Tests run on a model show that losses were less than 0.25 db inthe frequency range from 2 gHz. to 4 gHz. Isolation was 40 db at 2 gHz.falling smoothly to 21.5 db at 4 gHz.

In addition to being used as the input and output cOupler for anamplifier, the coupler of the present invention may also be used inimpedance bridges for microwave measurements and power monitoring. Forexample, if a radar transmitter is connected to port one, the antenna toport two, a microwave detector to port three, and a matched load to portfour, the power received in port three is proportional to the powerflowing from the transmitter to the antenna in the forward directiononly. Since the reflected wave from the antenna, if it exists, is notcoupled into port three, the detector monitors the power output of thetransmitter.

The coupler of the present invention may also be used as the couplingdevice for connecting a local oscillator to a mixer circuit. Forexample, in a strip line balanced mixer circuit, some means must beprovided for connecting the local oscillator signal to the mixercircuit. A coaxial line or waveguide type coupler obviously can not beused in an application such as this because of the incompatibility ofthe coaxial line and waveguide design with the strip line design.Heretofore, in strip line bal ance mixing circuits, coupling devices ofthe type illustrated in FIG. 3 were larger than the mixer itself. Withthe coupler of the present invention, the benefit of small size ismaintained.

While several embodiments of the invention, together with modificationsthereof, have been described in detail herein and shown in theaccompanying drawings, it will be evident that various furthermodifications are possible without departing from the scope of theinvention.

What is claimed is:

1. A four-port quadrature coupler comprising:

a plurality of strip lines in an interdigitated pattern, each of thefour ports of the coupler integral with at least one of said striplines, and

means for interconnecting alternate strip lines in the interdigitatedpattern.

2. A quadrature coupler as set forth in claim 1 wherein each strip linehas an effective length equal to a quarter wavelength of the bandwidthcenter frequency.

3. A quadrature coupler as set forth in claim 1 wherein saidinterconnecting means includes multiple crossover wires bonded toalternate strip lines to reduce the parasitic inductance.

4. A quadrature coupler comprising:

a dielectric substrate,

a plurality of strip lines adhesively secured to said dielectricsubstrate in an interdigitated pattern, each port of the couplerintegral with at least one of said strip lines, and

multiple crossover wires bonded to alternate strip lines forinterconnection thereof in an interdigitated pattern.

5. A quadrature coupler as set forth in claim 4 wherein said dielectricsubstrate is unglazed alumina.

6. A quadrature coupler as set forth in claim 4 wherein each strip linehas an effective length equal to a quarter of the wavelength of thebandwidth center frequency.

7. A quadrature coupler comprising:

a dielectric substrate,

a first strip line adhesively secured to said dielectric substrate andintegral with a first port of the coupler,

a second strip line adhesively secured to said dielectric substrate inan interdigitated pattern with the first strip line and integral withthe third port of the coupler,

a third strip line adhesively secured to said dielectric substrate in aninterdigitated pattern with the first and second strip lines andintegral with coupler ports two and four,

a fourth strip line adhesively secured to said dielectric substrate inan interdigitated pattern with said three strip lines, a first sectionof said fourth strip line integral with port two of the coupler, and asecond section of the fourth strip line integral with the port four ofthe coupler, and

means for interconnecting alternate strip lines in the interdigitatedpattern.

8. A quadrature coupler as set forth in claim 7 wherein saidinterconnecting means includes multiple crossover wires bonded toalternate strip lines to reduce parasitic inductance.

9. A quadrature coupler as set forth in claim 8 wherein said dielectricsubstrate is unglazed alumina.

10. A quadrature coupler as set forth in claim 9 Wherein each strip linehas a width of 4.5 mils and spaced from each other in an interdigitatedpattern by 3.0 mils.

11. A quadrature coupler as set forth in claim 10 wherein saiddielectric has a thickness of about mils.

12. A quadrature coupler as set forth in claim 11 wherein each stripline has an effective length equal to a quarter wavelength of thebandwith center frequency.

References Cited UNITED STATES PATENTS 3,162,717 12/1964 Lentz.3,332,039 7/1967 Oh 333-10 HERMAN KARL SAALBACH, Examiner P. L. GENSLER,Assistant Examiner US. 01. X.R 333-34 1

